The present invention relates to mixing an input signal with a local oscillating signal, and more particularly, to a mixer having a controllable load with reduced equivalent load value during switching transients of a local oscillating signal.
Wireless communication devices are widely used in a variety of fields. In general, each wireless communication has a transmitter for converting user data into RF signals for data transmission and a receiver for converting received RF signals into user data for data reception. In either of the transmitter and the receiver, a mixer plays an important role in transmitting/receiving the RF signal. Regarding the transmitter, the mixer mixes the input signal, either a baseband signal or an intermediate frequency signal, with a local oscillating (LO) signal to produce the RF signal to be transmitted. Regarding the receiver, the mixer mixes the RF signal received by an antenna with the local oscillating signal to produce either a baseband signal or an intermediate frequency signal.
More specifically, a CMOS-based mixer design is often employed to implement mixers inside the transmitter and the receiver of a wireless communication device. One problem known for CMOS-based mixers is the undesired low-frequency flicker noise originated from the noisy transistors and presented at the mixer output as interference, which degrades the mixer performance greatly. Some conventional solutions have been developed and applied to CMOS-based mixers, especially active mixers, to alleviate or eliminate the interference caused by flicker noise mixed with the local oscillating signal during switching transients of the local oscillating signal, i.e., at or around the zero-crossing of the local oscillating signal. For example, dynamic current injecting, dynamic current quenching, or dynamic current switching is applied to typical active mixers to block DC current from passing through transistors switched by local oscillating signal during the switching transients. However, additional circuit components added to the typical active mixer will induce static or dynamic DC offset current flowing to the transistors switched by local oscillating signal, resulting in LO leakage at the transmitter and second-order intercept point (IP2) at the receiver. Removing these unpredictable DC offsets is difficult, even though a predetermined calibration mechanism is employed. Furthermore, aforementioned dynamic current injecting/quenching/switching technique is applicable to active mixers only. In other words, regarding the passive mixers having no DC current, the conventional dynamic current injecting/quenching/switching technique fails.